remove unuseful content (#18644)

Author: zhangshikang

README.md

@@ -296,4 +296,3 @@ Contact us
 [9]: http://discuss.cocos2d-x.org "http://discuss.cocos2d-x.org"
 [10]: http://www.twitter.com/cocos2dx "http://www.twitter.com/cocos2dx"
 [11]: http://t.sina.com.cn/cocos2dx "http://t.sina.com.cn/cocos2dx"
-[12]: https://webchat.freenode.net/ "https://webchat.freenode.net/"